The invention relates to an exhaust gas turbocharger for an internal combustion engine of the type having a turbine wheel in a turbine housing, which turbine wheel is driven by engine exhaust gas and is connected via a shaft to a compressor wheel in a compressor housing, and a turbine guide wheel arranged upstream of the turbine wheel and mounted rotatably relative thereto and having a speed which can be varied as a function of engine load.
In an exhaust gas turbocharger disclosed in the prior German Published Patent Application DE 4 011 818 it is provided to fix the guide wheel concentrically surrounding the turbine wheel in the low to high part load range and to feed a portion of the exhaust gas energy in the high load ranges back into the crankshafts of the internal combustion engine (compound drive) via this guide wheel, which yields a clear improvement in the efficiency of the internal combustion engine. However, in the case of a fixed turbine guide wheel a relatively steep approach flow onto the turbine wheel takes place depending on the blading of the guide wheel, resulting in a relatively low turbine wheel speed and thus a low turbine output. This has a disadvantageous effect on the response of the exhaust gas turbocharger or of the internal combustion engine after a positive load change.
It is known from "Automotive Engineering, September 1985, pages 70-72" to provide in an exhaust gas turbocharger a permanently fixed turbine guide lattice whose blades are set in the low load range, resulting in a flatter approach flow onto the turbine wheel and thus a higher turbine speed in these operating ranges. This setting of the blades is, however, bound up with a reduction in the flow cross-section, so that in these operating states there are high friction losses, which have a negative influence on the efficiency of the exhaust gas turbocharger and thus, in turn, on the overall efficiency of the internal combustion engine. The adjustment of the guide blades is performed via a mechanically operating adjustment apparatus whose components are partly arranged likewise in the turbine housing. The guide apparatus is therefore subjected to a high thermal load.
It is therefore an object of the invention to provide an exhaust gas turbocharger of the type referred to above, with which the response of the internal combustion engine after a positive load change can be improved without any impairment of the overall efficiency of the internal combustion engine.
The object is achieved according to the invention by providing an arrangement wherein at least one braking device serves to reduce the speed of the guide wheel in higher load ranges.
The invention ensures that a relatively high turbine wheel speed is reached even in the part load range, so that the exhaust gas turbocharger can be maintained at a relatively high speed. Consequently, after a positive load change the exhaust gas turbocharger very quickly reaches its speed corresponding to the new load value, so that the output corresponding to this stipulated load is also delivered by the internal combustion engine just shortly after the change in the stipulated load. Owing to the fact that the approach-flow direction in the exhaust gas turbocharger according to the invention is influenced not by a change in position of guide blades but by a change in speed of the guide wheel, the change in the approach-flow direction onto the turbine wheel is not bound up with a change in the flow cross-section. In other words, this means that the relatively high turbine speed is reached in the part load range without an appreciable reduction in the flow cross section, so that there are also no additional friction losses reducing the efficiency of the exhaust gas turbocharger or the overall efficiency of the internal combustion engine.
Moreover, in a manner completely opposite to a conventional exhaust gas turbocharger having adjustable turbine geometry, the exhaust gas turbocharger according to the invention also enables impulsive charging (two-duct turbine housing), because here the guide blades are arranged fixed on the guide wheel and thus capable in terms of strength of withstanding the increased stress of impulsive charging.
A further advantage of the exhaust gas turbocharger according to the invention is to be seen in that the device for braking the turbine guide wheel can be arranged in the region of colder zones, so that said device also is not prematurely threatened with wear because of too high thermal charging.
The heat produced during a braking operation can be dissipated directly via the cooling water of the internal combustion engine, so that after a cold start of the internal combustion engine its warm-up phase can be shortened. The quantity of heat produced during a braking operation can also be reduced if, as proposed in certain preferred embodiments of the invention, the turbine guide wheel is connected or coupled to a compressor guide wheel concentrically surrounding the wheel of the compressor, which compressor guide wheel has a blading opposite to that of the turbine guide wheel. Such an opposite blading has the effect that starting from a specific limit value for the internal combustion engine load the set of guide wheels, that is to say the unit composed of compressor guide wheel and turbine guide wheel, is ever more strongly braked with increasing load. Given an appropriate design of the turbine guide wheel blading and compressor guide wheel blading, it is even possible to eliminate a further braking device according to certain contemplated embodiments.
Moreover, a compressor guide wheel surrounding the compressor wheel acts like a second compressor stage and thus increases in an advantageous way the pressure difference in the quantity of air throughput.
The use of a hydraulic retarder as braking device has the advantage that not only can the heat produced by the braking of the turbine guide wheel be dissipated via the hydraulic fluid led off from the hydraulic retarder during the control process--the hydraulic fluid can likewise be used to cool the components heated by the exhaust gas such as, for example, bearing housing, bearings, etc. If, instead of a hydraulic fluid, the engine oil is applied to the hydraulic retarder, the additionally produced quantity of heat can be guided into the cooling water indirectly via the heat exchanger customary in internal combustion engines. An external heat dissipating device can be entirely eliminated if instead of the hydraulic retarder use is advantageously made of a speed variator.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.